Generation and focusing of energy waves, such as acoustic waves (or shockwaves, the terms being used interchangeably throughout) for purposes of medical treatment such as lithotripsy (stone fragmentation) or orthopedic treatment are accomplished through a variety of methods. Each method incorporates acoustic wave generation and associated focusing apparatus.
The prior art may be classified according to the geometry of the acoustic wave generation and associated focusing:
a. Point source and ellipsoidal reflector: A point source typically comprises electrohydraulic apparatus. Fast discharges of electrical energy between tips of closely spaced electrodes give rise to a sequence of spherical waves in a propagation liquid. The electrodes are arranged with respect to an ellipsoidal reflector, which has two focal points. The electrical energy is discharged at the first focus, and the waves are focused onto the second focus.
b. Planar source and acoustic lens: A planar source typically comprises electromagnetic apparatus. A thin circular membrane applies pressure to the propagation liquid by being jolted or repelled away from a planar coil. Fast discharges of electrical energy into the coil and the associated rapid changes in the magnetic field induce currents in the membrane, turning it into a magnet with a polarization opposite to that of the coil. The ensuing repulsions of the membrane, which is in close contact with the propagation liquid, generate the acoustic waves. The waves are then focused by a lens to a target located at the focus of the lens.
c. Cylindrical source and parabolic reflector: The cylindrical source generates an acoustic wave that emanates radially outwards from the longitudinal periphery of the cylinder. For example, a coil may be mounted on a cylindrical support and a cylindrical membrane. The coil may be pushed or repelled radially, gives rise to outwardly propagating cylindrical waves. A parabolic reflector focuses the waves into a point on the cylindrical axis of the system. It is noted that the cylindrical source of the waves (also referred to as the cylindrical wave transducer) is inside the parabolic reflector.
d. Spherical source: Spherical waves may be generated by an array of piezoelectric transducers or by an electromagnetic approach with a spherical membrane being repulsed inwardly into the propagation liquid. No further focusing is required.